(1) Field of the Invention
The present invention relates to a system and method for controlling a damping force characteristic of a shock absorber (hereinafter referred to as a damper) installed between an unsprung mass and a sprung mass of an automotive vehicle and particularly relates to the controlling system and method with a countermeasure against noise signals.
(2) Description of the Background Art
Japanese Patent Application First (unexamined) Publication No. Showa 61-163011 exemplifies a previously proposed damping force controlling system with the countermeasure against noise signals.
The disclosed damping force controlling system determines whether a direction of a velocity of the sprung mass coincides with a direction of a relative velocity between the unsprung mass and the sprung mass. The direction of the velocity of the sprung mass is detected by means of a detector for detecting the direction of the velocity of the sprung mass (vehicle body) and the direction of the relative velocity is detected by means of another detector for detecting the direction of the relative velocity.
The disclosed damping force controlling system carries out a basic control such that a control signal is output so as to reduce a damping factor of the damper when it is determined that the velocity and relative velocity do not coincide with each other and the control signal is output so as to increase the damping factor of the damper when it is determined that the velocity and the relative velocity coincide with each other.
Furthermore, the disclosed damping force controlling system determines whether an absolute value of the difference between a relative displacement between the sprung mass and unsprung mass and a neutral point of the relative displacement (a moving average in a long term of the relative displacement) exceeds a predetermined dead zone and switches the damping factor of the damper toward the higher damping side in the basic control described above only when determining that the neutral point exceeds the dead zone.
However, since, in the disclosed damping force controlling system, the dead zone is provided in a region in the vicinity to the neutral point, the control described above is possible with noise signals eliminated. Conversely, since the control is not performed until the detected signal value of the neutral point exceeds the dead zone, a delay of control correspondingly occurs. Thus, a timing at which the damping factor is switched becomes later than the timing at which the damper is in the neutral state.
Consequently, the steering stability of the vehicle deteriorates.